Organic Horticulture in the Mid-Atlantic/Weed Management

Weed Management

Weed control is at the core of gardening, since without it the garden would eventually revert to meadow, scrubland, and eventually forest. As with most garden chores, prevention and diligence are key, since weedy plants tend to grow much faster than ornamentals and crops.

Long-term control requires three steps when engaging in weed control: knowing the weeds, removing the weeds, and preventing the re-emergence of the weeds.

Like cultivated plants, each species of weed has its unique optimal growth conditions, habit, and life cycle, so being able to identify the weeds is essential for achieving effective control. Some weeds are easily controlled within a season, while others may require a longer-term commitment, but knowing how a weed spreads and reproduces enables the gardener to come up with a management plan for each type.

For professional gardeners, learning the common weeds found in the region is both easier and more useful than knowing the cultivated plants, since most gardens will only have perhaps 20 species of weed, while on the other hand having many more species and cultivars of desired plants. Knowing which plants should always be removed -- and knowing the best way to remove and dispose of them -- makes it much easier to simply jump into a garden and clean it up, rather than needing a "tour" of each new site to see what the "good plants" are.

The simplest distinction that needs to be made is whether a weed is an annual (growing from seeds each year), or perennial (growing from persistent roots or woody parts). Annual weeds can in most cases be controlled with greater ease, since one needs only interrupt the reproductive cycle, while perennials need to be controlled at the root as well as breaking the reproductive cycle.

Perennial weeds tend to have more substantive root systems, and most will grow back if at least the vast majority of the root is not removed or otherwise destroyed. Serious infestations of rhizomatous or stoloniferous plants (such as canada thistle, mugwort, nutsedge, or bindweed) may require soil barriers to smother the plants over a season.

Annual weeds, on the other hand, tend to be much easier to pull, but also produce very large amounts of seed, since they do not use any energy on building up reserves of starches to survive winters (or summers, in the case of winter annuals).

Biennial weeds are somewhere in between, since they spend their first year building up starches, and then use up all these starches at the end of their life cycles to produce seeds.

The second most important thing to know is when the seeds are produced and how they are spread. Removing weeds after they have gone to seed does nothing to prevent further germination (which can be an even greater problem when herbicides are used, since even after the weed appears to be "dead", the seeds can in many cases continue to develop). Grubbing before flowers are produced is the best time. If flowers are already present, the weeds should be treated as if seeds were present when deciding on a disposal method.

Knowing the type of root that a weed has, how firmly it is attached to the above ground parts, as well as how much of the root is needed for the plant to re-grow is important for proper, long-term control. Weeds with taproots or woody roots require digging tools for removal, while plants with shallow or noncomplex root systems can often simply be pulled by hand. See the chapter on weeding tools.

All weeds spread by seed, and some perennials also spread vegetatively (through roots, stolons, etc.). How seeds are transported from the parent plant to the garden is important to understand, particularly since some weeds are easily spread on feet or on tools.

Weeds that spread vegetatively are often the most intractable, since stoloniferous and rhizomatous weeds often grow back from even the smallest root fragments. Such weeds can best be controlled by soil barriers (such as newspaper and mulch), by removing and replacing the soil outright, or when those methods are simply not an option (for example, when perennial weeds are growing among desirable plants that cannot be transplanted away), control can be achieved after a time by diligent and regular grubbing.

Weeds that spread by seed can often be managed with a preventative approach. The most effective preventative is simply ensuring that the soil is always covered by a layer of seed-free mulch, since this will prevent any seeds on the soil surface from being exposed to light, which is the signal that causes many seeds to germinate. For this same reason, any time the soil is disturbed (by grubbing weeds, planting, or any other reason), fresh mulch should be added to cover up the disturbed area, since weed seeds will have been brought to the surface.

Weeds can also be controlled using Corn Gluten Meal (CGM), which is a byproduct of corn processing. While this only works for smaller seeds, it is an excellent choice for lawns and other areas (such as vegetable gardens) where mulches can't be used. CGM breaks down into nitrogen and other fertilizers after 4-6 weeks, after which it must be re-applied.

Seed dispersal methods vary from plant to plant. Many plants rely on birds or other animals to disperse the seeds. Others are spread by wind. Finally, many are spread by foot traffic, tools, lawn mowers, etc.

Birds spread seeds by eating the fruits, then passing the seeds a short while later. Bird-spread weeds are thus most common around the edges of shrubs, under power lines or other places where the bird perches. Little can be done to prevent birds from spreading these seeds, so the soil underneath regular perching spots should simply be kept well-mulched or otherwise shaded (by dense ground covers, etc.) to prevent germination, and regularly patrolled for seedlings (particularly tree seedlings) that are able to germinate despite preventative measures.

Nut-producing trees generally rely on squirrels and other rodents to plant the seeds deep in the soil. Neither mulch nor CGM provides any control to these plants, since their germination is triggered by soil temperature and moisture. Discouraging squirrels or removing any nearby nut-producing trees are the only preventatives. Weeds of this group need to be pulled as soon as they are seen, since most of them quickly develop deep taproots that make larger plants very difficult to remove.

Wind-spread seeds end to settle against windbreaks or in low areas. The germination of smaller seeds can be prevented by either CGM or diligent mulching. Larger seeds (such as maple, ash, tuliptree, etc.) should be blown off after they fall, before additional mulch is laid down.

Finally, weed seeds are also found in the soil itself, lying dormant under the surface (these seeds are known as the "soil seed bank"). This is why it is important to always add fresh mulch after weeds are removed, because once these dormant seeds are exposed to sunlight, they will germinate almost immediately.

Many of the most common weeds of the region produce very small seeds that are dispersed on feet, tools, and other equipment. While these weeds are among the most numerous found in the field, their numbers can be drastically reduced by proper hygiene in the garden. When pulling these weeds, hey should be immediately put into a container (never left on the lawn, soil, or even on hard surfaces), tools should be wiped down after working near them or in soil where they are present, and shoes should be wiped or hosed off when moving from a weedy area to an area already cleaned.

With all weeds, a key management strategy is to prevent seeds from being produced whenever possible. For example, nearby waste areas, unkempt woodlands, or invaded wildflower meadows should be monitored for problem weeds, and where possible mown or cleared before the seeds ripen. In many cases it is certainly worthwhile to eradicate seed sources even on neighboring properties (even just mowing or trimming weeds before the seeds ripen).

Once the weeds found on a site have been identified, the next step is to have the right tools ready, prepare for dealing with the waste stream, and finally make sure the site is ready to be worked on.

Tools include a hand cultivator to loosen soil around the roots, a hoe for larger areas that have only seedlings, a tote to put the weeds in (or two totes, if using "split-stream" disposal), pruners for tree seedlings that are too well-established to pull out effectively, and finally a pair of gloves for removing spiny, thorny, or poisonous plants.

For disposal a compost area should be kept ready to accept materials. If using "hot" composting, all materials can go to the same place. If a cold compost system is being used on-site, seed-bearing materials must be kept separate in order to prevent further dispersal when the compost is used in the garden.

To prepare the site, make sure the edges are well-defined, if possible make sure the lawn was recently mown, and (again, if possible) the soil should me moist but not wet to make roots easier to remove.

Finally, a supply of mulch should be on-hand to cover any areas of soil that are disturbed during the process.

Once the weeds are identified and the tools and site are prepared, it's time to start grubbing them out. Different weed species require different removal methods, but the control method will also depend on where they are growing, the density of the desired plantings, etc.

In large, open areas where there are not currently any desirable plants, hoeing (particularly with stirrup hoes or offset hoes) is often the most efficient way to get the weeds out. If some plants are unable to be hoed, follow up with a mattock-type cultivator or if necessary with a spade for deeply tap-rooted plants. When hoeing, always follow up immediately by raking off the area, and then adding a layer of mulch.

In areas that are more densely planted, use the claw part of a mattock-type cultivator to loosen the soil below the weed by hitting just behind it and prying gently. This allows the root system to be removed intact, and is especially important for weeds that spread by rhizomes or are easily broken away from the root system when pulling. Mulch areas where the soil has been loosened immediately in order to prevent any newly-exposed soil seeds from germinating.

If the soil is soft and moist enough, simply manually pulling is often the easiest way. When pulling by hand, use a slow, firm motion rather than trying to jerk the weeds out, since this will more often than not cause the top of the weed to simply break off of the root. If gloves are worn, make sure the gloves fit very tightly so that the gardener can grab the plant just beneath the soil surface at the crown, rather than trying to pull by the above-ground stems. Again, add mulch to prevent new weeds from germinating.

If weeds are growing within the crowns of desirable plants, first try simply pulling, making sure to grab just below the soil line and also making sure that the weed (not the garden plant) is what's being pulled.

In more extreme cases (such as when lawn grass or other grasses get into a perennial forb), it may be necessary to dig up the plant, separate out the weed roots (bare-rooting may be necessary), and replant. Bear in mind that the perennial will be stressed in the process, and may need shelter from direct sunlight for a few days, and may need extra irrigation for several weeks if this is done in summer. Alternatively, simply snip the weeds at the soil line and return regularly, putting off digging until fall when the plant will not be as badly stressed.

In all cases, remember to put the weeds directly into a container for disposal, since putting the weeds on soil, lawn, or other surfaces will give the plant an opportunity to drop its seeds as it dries out, and risks leaving perennial root fragments behind that may regrow under the new mulch.